In recent years, demand for improvement in thermal resistance of acrylic resins for use in light electrical appliances and industrial machine parts, in particular parts of vehicles, has been increasing.
Imidization of methacrylic polymers is known as a conventional method for improving the thermal resistance of acrylic resins. For example, Japanese Patent Registration No. 1363497 discloses a process, for producing imidized acrylic resins that contain imide units and are soluble in DMF, which comprises reacting an acrylic resin with ammonia or a primary amine in an extruder at about 200.degree. C. in the substantial absence of water. This process however comprises using an expensive extruder of special structure and maintaining the condition of substantial absence of water, which require complex operation, and further has the problem that the product obtained tends to be of non-uniform composition, when producing partially imidized acrylic resins. Furthermore, even when full attention is paid to maintain the condition of substantial absence of water, the obtained product inevitably contains by-product acid and acid anhydride, thereby decreasing its fluidity and increasing its water-absorptive capacity.
Japanese Patent Application Laid-open No. 20904/1985 discloses a process for producing polyglutarimide which comprises condensing polymer or copolymer of methacrylamide with a methacrylate. This process does not take into consideration the fluctuation of copolymerization composition caused by a large difference between the reactivity of methacrylamide and that of methacrylate, and hence yields only polyglutarimides with high haze value. In addition, this process comprises condensing copolymer of methacrylamide by heating at 150.degree. to 300.degree. C. in the absence of a solvent, and is reported to be incapable of giving a solvent-soluble polymer with a copolymerization ratio of methacrylamide exceeding 30% by weight because of occurrence of inter-molecular condensation, which is attributable to this treatment. Furthermore, the condensation reaction must be conducted at high temperature for a long period of time, which is very disadvantageous from the viewpoint of commercial production.
Japanese Patent Application Laid-open No. 191815/1988 discloses a process which comprises transforming copolymers of acrylate with (meth)acrylamide into polymers containing glutarimide by imidization in an organic solvent in the presence of an acidic compound. Since this process uses an acidic compound as an activator for imidization reaction, it requires a large amount of the acidic compound, high reaction temperature and long reaction time. Where in practice an acidic compound is used as a promoter or activator of imidization reaction, in such a trace amount as with catalysts in general, the reaction never proceeds smoothly. This process is therefore very disadvantageous when used for commerical production.
Under these circumstances, there has not been proposed any process for producing advantageously on an industrial scale, in particular, glutarimide copolymers having N-hydrogenglutarimide component and having high properties including excellent transparency.